The invention relates to a method of inserting a drainage wick into the ground and to a device for anchoring the drainage wick.
Drainage wicks are commonly used for carrying out vertical draining in the ground in order to improve the strength of ground and generally consist of paper material in the form of strips or of a band-shaped plastic core enclosed in a suitable filter material. Such drainage wicks are widely used to consolidate soft compressible soils (such as clays and/or fine grained soils). Consolidation of a compressible soil occurs as pore water is squeezed from the soil matrix. The installation of vertical drains provides shortened drainage paths for the water to exit the soil and thus can reduce significantly the settlement time. In order to insert such a drainage wick in the ground, for example to a depth of 10 m, up to 40 or 60 m, the drainage wick is threaded through an inserting tube driven into the ground. When such an inserting tube is extracted from the ground, the drainage wick remains in the soil, the soil pressurizes against the drainage wick, and water may travel through the permeable filter material of the drainage wick along the plastic core.
Such a drainage wick inserting method is disclosed in U.S. Pat. No. 2,577,252 and devices for inserting a drainage wick are known from U.S. Pat. No. 3,891,186 or U.S. Pat. No. 4,755,080. According to prior art methods, the drainage wick is threaded through the inserting tube and an anchorage component, such as a plate or bar, is attached to the lower end of said drainage wick. The drainage wick and anchorage component are then driven downwardly into the ground by means of said inserting tube. The inserting tube is then pulled up whereby the anchorage component and drainage wick are permitted to remain in the ground.
To avoid the anchorage component not remaining at its bottom position when the inserting tube is pulled, the drainage wick and the anchorage component are frequently driven downwardly deeper, to a hard soil layer. The anchorage component is then easily retained in the hard soil layer and the drainage wick is maintained in its bottom position. Such a solution is commonly used, but presents drawbacks. For example, such a process may be unusable due to a geological constraint, such as aquifer presence in the hard soil. It is then necessary to avoid communication between the different soil layers to prevent water contamination.
The same process should also be avoided when using vacuum soil consolidation as disclosed for example in U.S. Pat. No. 6,254,308. Since suction means are used to evacuate the liquid collected from the ground, the draining process would be endless if drainage wicks were to be maintained in contact with underlying permeable soils. Similar problems may be encountered when a hard soil layer, namely an aquifer including layer, is included between two soft soils layers, which need to be drained.